The present invention relates to light modulation systems in general, and more particularly to an electro-optical system for electronically controlling the modulation of a randomly polarized light beam.
Light valves or modulators utilizing electro-optical crystal elements for modulating linearly polarized light beams are generally well known as evidenced by the U.S. Pat. Nos. 3,400,992 issued to McNaney; 3,767,287 issued to von Hundelshausen; 3,823,998 issued to Yazaki; 3,879,687 issued to Daehlin et al.; and 3,910,680 issued to Kakeda. Moreover, examples of improved aspects of proposed optical systems of this type which have been applied to laser apparatus may be found in the U.S. Pat. Nos. 3,429,636 and 3,713,032, both issued to the same inventor and assigned to the same assignee as the present application. The electro-optical system of U.S. Pat. No. 3,713,032 permits modulation of the output light beam coupling of a laser by electronically controlling the voltage potential applied to the electro-optical crystal cell included therein. And, the electro-optical system of U.S. Pat. No. 3,429,636 provides for light modulation with two electro-optical crystal cells aligned in tandem along the optical axis to provide for a lower modulation voltage potential for inducing birefringence therein.
Apparently, all of the aforementioned systems are directed to the modulation of linearly polarized light. In some cases, especially when modulating light in a laser optical cavity, it is preferred that randomly or unpolarized light be used during the resonating cycle. For this reason, other electro-optical modulation configurations have been proposed. One such proposed system, for example, is disclosed in the U.S. Pat. No. 3,719,414 issued to the same inventor and assigned to the same assignee as the present application.
More specifically, with regard to a system of the type described in U.S. Pat. No. 3,719,414, a birefringent crystal, preferably calcite, an electro-optical crystal, and another birefringent crystal may be aligned along an optical axis in a tandem configuration. A randomly polarized light beam, projected along the optical axis and incident to the first birefringent crystal, may be split into two orthogonal component rays as it passes through the first birefringent crystal. The electro-optical crystal, in the unenergized state, passes the orthogonally polarized rays unaltered to the second birefringent crystal which is configured to recombine the orthogonally polarized rays to reform the randomly polarized beam along the optical axis at the output thereof. At times when the electro-optical crystal is energized, the orthogonally polarized rays passing therethrough may be polarization modulated such that the respective polarization axes are rotated by 90.degree., for example, and then passed through the second birefringent crystal. As a result of the 90.degree. rotation of the plane of polarization, the rays are no longer recombined at the output of the second birefringent crystal, but are spatially separately, generally along paths which are parallel and equidistant from the optical axis at the output of the second birefringent crystal.
When this principle of birefraction modulation is applied to a laser optical cavity, for example, the second birefringent crystal may be replaced with a totally reflective mirror as depicted in the U.S. Pat. No. 3,719,414. In this configuration, the reflected orthogonally polarized beams from the mirror may be passed back through the electro-optical crystal and depending on the state of said crystal the two beams may either be recombined along the optical axis as they pass through the birefringent crystal and thereafter may be returned to a laser rod from which they were emitted compositely as a randomly polarized beam or the two beams may remain separated as they pass through the birefringent crystal and may be directed away from the laser rod.
Thus, the operation of this type of electro-optical system is one of Q-switching a randomly polarized light beam in a laser optical resonant cavity. That is, when the electro-optical crystal is unenergized, the randomly polarized light beam may be reflected and returned to its laser source in a high Q transmission state and upon energization, the light beam is diverted from its laser source resulting in a low Q transmission state. Note that the operation of the electro-optical system is independent of the light beam polarization.
In these systems, the birefringent crystals, which offer the characteristics of double refraction to the orthogonally polarized component rays of a randomly polarized light beam, are presently very expensive. In the case in which calcite is used as the birefringent crystal, the present cost may be as high as $7,000.00 per crystal needed. In addition, the electro-optical crystal utilized therein must be made of a physical size that its aperture be capable of accommodating the outside dimensions of the two parallel rays of light provided thereto from the birefringent crystal. Moreover, either a mirror or an additional birefringent crystal is required to provide reflection or recombination, respectively, of the orthogonally polarized component rays of the light beam. In addition to these expenses, the formation of the orthogonally polarized light rays provide undesirable thermal optic effects in the crystal elements which may cause non-uniform heating therein, that is, heat in being generated in two places primarily in the areas where the two beams of light are concentrated.
In conclusion, it appears from the viewpoint of expense, performance and configuration, that a system more compact, in terms of alignment, using less costly elements and providing for the same or better performance by offering a similar function of modulating a randomly polarized light beam would provide a viable improved alternative to the present type systems. Thus, it is the intention of the specification here following to describe such an embodiment for the electro-optical modulation of a randomly polarized light beam.